HIGH-CARBON STEEL. . 95
A third variation is the melting of wrought-iron with- a proper proportion of pig to raise the carbon to the desired point, while in still another, used in Sweden, the charge of the crucible consists of pig and iron ore. The aim of all methods is to obtain a malleable metal containing from .60 to 1.40 per cent, carbon, and free from blowholes. For certain purposes some special element like chromium, or tungsten, may be used as an alloy, but with this exception every other ingredient may be regarded as an impurity.
SEC. Vb.—Chemical reactions in the crucible.—The best tool steel must be as tough as possible, and, therefore, the phosphorus should not be over .02 per cent. Sulphur, which does not appreciably affect brittleness, but does decrease forgeability, is not so important, but should not exceed .04 per cent. Manganese may be in larger quantity, and it is not uncommon to put into the pot a mixture o>f manganese ore and carbon so that metallic manganese may be reduced. If the percentage does not exceed .20 it has little bad effect; if much above this, it will cause brittleness and liability to crack in quenching.
Just after the steel is melted'there is more or less action in the crucible. In addition to the iron and charcoal in the pot, there is a small amount of glass or similar material to give a passive slag; also a little air, some slag and oxide of iron, the scale and rust on the surface of each piece of metal, and silica, alumina and carbon from the scorification of the walls. A little time is necessary for the various.reactions to occur and for the reduction of silicon from the slag and lining in accordance with the following equation:
The carbon is drawn either from the charcoal, from the metal, or from the crucible. In the case of graphite pots the supply from the latter source will be ample, while even clay pots furnish quite an amount from the coke which is mixed with the clay. This reduction goes on until the steel contains from .20 to .40 per cent, of silicon and the metal lies quiet and "dead," when the pot is taken from the furnace and the contents cast into ingot form. The crucible lasts from four to six heats, and the weight of a melt is about '80 pounds when the crucible is new.
SEC. Vc.—Chemical specifications on high steel.—In olden